Before getting into this section it is important to draw attention to the difference between characters and glyphs.

A character is a semantic unit representing an indivisible unit of text in memory.

A glyph is the visual representation of a character or sequence of characters.

The example on the slide shows two glyphs for the single character a, and two glyphs for a single character Han character. This distinction will become very important in this section. For more information about the distinction between characters and glyphs, see Unicode Technical Report #17.

A font, by the way, is a collection of glyphs.